Fuel cell systems include a fuel processor and a fuel cell stack. The fuel cell stack produces an electric current from the product stream of the fuel processor. For example, the fuel processor may produce hydrogen gas or hydrogen-rich gas from common feed stocks, such as water, a carbon-containing feedstock, or both. The fuel cell stack produces an electric current from the hydrogen gas.
An example of a conventional fuel cell system is shown in FIG. 1 and indicated generally at 10. System 10 includes a fuel processing assembly 11 and a fuel cell stack 22. Fuel processing assembly 11 includes a suitable fuel processor 12 and a feed stream delivery system 17, which delivers a feed stream 16 to the fuel processor. Fuel processor 12 is adapted to produce a product hydrogen stream 14 containing hydrogen gas from feed stream 16, which contains the feedstock for the fuel processor.
The composition and number of individual streams forming feed stream 16 will tend to vary depending on the mechanism by which fuel processor 12 is adapted to produce product hydrogen stream 14. For example, if fuel processor 12 produces stream 14 by steam or autothermal reforming, feed stream 16 contains a carbon-containing feedstock 18 and water 20. If fuel processor 12 produces stream 14 by pyrolysis or catalytic partial oxidation of a carbon-containing feedstock, feed stream 16 contains a carbon-containing feedstock and does not include water. If fuel processor 12 produces stream 14 by electrolysis, feed stream 16 contains water and does not contain a carbon-containing feedstock. Examples of carbon-containing feedstocks include alcohols and hydrocarbons. When the feed stream contains water and a carbon-containing feedstock that is soluble with water, the feed stream may be a single stream, such as shown in FIG. 1. When the carbon-containing feedstock is not miscible in water, the water and carbon-containing feedstock are delivered in separate feed streams, such as shown in FIG. 2.
Fuel cell stack 22 is adapted to produce an electric current from the portion of product hydrogen stream 14 delivered thereto. Fuel cell stack 22 includes a plurality of fuel cells 24 integrated together between common end plates 23, which contain fluid delivery/removal conduits (not shown). Examples of conventional fuel cells include proton exchange membrane (PEM) fuel cells and alkaline fuel cells. Fuel cell stack 22 may receive all of product hydrogen stream 14. Some or all of stream 14 may additionally, or alternatively, be delivered, via a suitable conduit, for use in another hydrogen-consuming process, burned for fuel or heat, or stored for later use.
Fuel cell stack 22 receives at least a substantial portion of product hydrogen stream 14 and produces an electric current 26 therefrom. This current can be used to provide electrical power to an associated energy-consuming device 28, such as a vehicle or a house or other residential or commercial dwelling.
In FIG. 3, an illustrative example of a fuel cell stack is shown. Stack 22 (and the individual fuel cells 24 contained therein) includes an anode region 32 and a cathode region 34, which are separated by an electrolytic membrane or barrier 36 through which hydrogen ions may pass. The regions respectively include anode and cathode electrodes 38 and 40. The anode region 32 of the fuel cell stack receives at least a portion of product hydrogen stream 14. Anode region 32 is periodically purged, and releases a purge stream 48, which may contain hydrogen gas. Alternatively, hydrogen gas may be continuously vented from the anode region of the fuel cell stack and re-circulated. The purge streams may be vented to the atmosphere, combusted, used for heating, fuel or as a feedstock for the fuel processing assembly. The purge streams from the fuel cell stacks may be integrated into a suitable collection assembly through which the combined purge stream may be used for fuel, feedstock, heating, or otherwise harvested, utilized or stored.
Cathode region 34 receives an air stream 42, and releases a cathode air exhaust stream 44 that is partially or substantially depleted in oxygen. Air stream 42 may be delivered by an air delivery system 46, which is schematically illustrated in FIG. 3 and may take any suitable form, such as a fan, blower or the like. Electrons liberated from the hydrogen gas cannot pass through barrier 36, and instead must pass through an external circuit 49, thereby producing electric current 26 that may be used to meet the load applied by device 28. Current 26 may also be used to power the operation of the fuel cell system. The power requirements of the fuel cell system are collectively referred to as the balance of plant requirements of the fuel cell system.
Because fuel cell system 10 relies upon a single fuel cell stack and a single fuel processor, it suffers from some limitations due to its reliance on those components. For example, if stack 22 requires maintenance, is damaged or otherwise needs to be removed from service, system 10 is unable to provide power to device 28, other than previously stored power, if any. Similarly, if fuel processor 12 requires maintenance, is damaged or otherwise needs to be removed from service, system 10 is unable to provide feedstock, such as product hydrogen stream 14, to fuel cell stack 22, other than previously stored feedstock, if any.